The activated sludge process has been used for many years for the removal of biochemical oxygen demand, BOD, from the wastewater. This process consists of maintaining an aeration basin in which wastewater is fed to a suspension of microorganisms to form a mixed liquor. The mixed liquor is aerated to furnish oxygen for the respiration of the biomass, which sorbs, assimilates, and metabolizes the biological oxygen demand of the wastewater.
After a suitable period of aeration, the mixed liquor is introduced to a clarifier in which the biomass settles and the treated wastewater overflows into the receiving stream. A major portion of the settled biomass, which is concentrated at the bottom of the clarifier, is recycled to the aeration basin and a minor portion is purged in order to maintain a constant biosolids inventory within the system. This process has been extensively described in patents and technical publications. While the earlier commercial activated sludge plants employ atmospheric air to furnish the oxygen required to sustain the metabolic function of the microorganisms developed in the activated sludge, more recent commercial installations employ in one or more of the oxidation stages an aeration gas having a higher oxygen content than the 21% contained in atmospheric air.
While effective removal of the organic matter (BOD) present in wastewater has been achieved by the conventional activated sludge processes, there remained problems due to the retained presence of nitrogen and phosphorus values in the "purified" waters from such conventional processes, resulting in eutrophication of the waters to which they were returned. Various methods were devised for the removal of phosphorus and/or nitrogen components before return of the treated waters to holding reservoirs or natural bodies of water. Some of these methods are described by Shindala, A., in Water and Sewage Works, June 1972 at pages 66 to 71 and July 1972 at pages 60-67.
In U.S. Pat. No. 4,056,465 a modified activated sludge system is disclosed wherein BOD-containing wastewater and recycled sludge are initially admixed under anaerobic conditions in the substantial absence of oxygen or oxidizing agents and subsequently subjected to aeration and clarification. By the therein disclosed operation effective removal of phosphates is stated to be obtained while favoring the selective production of non-bulking biomass. In an alternative modification described in the patent, nitrates and nitrates (designated NO.sub.x -) are also removed by interposing an anoxic treating zone between the anaerobic zone and the aerating zone. The patent advocates that the initial admixture of the recycled biomass (sludge) with the wastewater influent be under anaerobic conditions such that the basin or zone in which the mixed liquor is first formed is substantially free of NO.sub.x - and contains less than 0.7 ppm dissolved oxygen (DO); and that during the aeration or oxygenation for removal of BOD from the mixed liquor there be maintained a dissolved oxygen content of at least 1 ppm. In the modification described for removal of NO.sub.x - as well as phosphates, the interposed anoxic treating zone has a DO content not in excess of 0.7 ppm and NO.sub.x - is admitted to that zone, obtained by internal transfer of mixed liquor thereto from the oxygenated zone, to provide a NO.sub.x - concentration in excess of 2 ppm (expressed as elemental nitrogen). Under these conditions in the anoxic zone the NO.sub.x - is reduced to elemental nitrogen gas and discharged.
Systems such as those described in U.S. Pat. No. 4,056,465 employing an anaerobic zone for initial admixture of influent wastewater with recycled biomass have become to be known as "A/O" systems. The modified systems having an anoxic zone interposed between the anaerobic and the oxic treating zones are referred to as "A/A/O" systems or "A.sup.2 /O" systems.
Further improvements in systems of the A/O and A.sup.2 /O type are set out in U.S. Pat. No. 4,271,026. According to that patent enhanced removal of phosphorus is obtained by maintaining operating conditions within the processing system encompassing the initial anaerobic treatment and extending through the process up to, but not including, the separation step, a BOD to phosphorus (BOD/P) ratio from about 5:1 and up to about 50:1, wherein BOD is expressed as milligrams of soluble BOD (exclusive of that attributable to ammonia) per liter of influent, and P is soluble phosphate expressed in milligrams of elemental phosphorus per liter of influent. Also the system is to be operated at a food to biomass (F/M) ratio from about 0.09 to an upper limit of about 1.4, wherein F is the total weight of soluble BOD introduced into the process per 24 hour day, and M is the weight of volatile suspended solids in the process system.
In an article by Davelaar et al, titled "The Significance of an Anaerobic Zone for the Biological Removal of Phosphate from Wastewaters", (Water, S. A. vol. 4, No. 2, April 1978, pages 54 to 60), the authors describe certain of the theories advanced with respect to the function of the anaerobic zone, particularly with respect to phosphate removal, and the adverse effect on phosphate removal of the presence of nitrate in the inflow to the anaerobic zone. The article compares experimental results obtained in the laboratory scale operation of two activated sludge units, designated A and B. In the B unit the sewage influent and recycled sludge were admixed in an anoxic zone and flowed therefrom to an aerobic zone, followed by solids separation to recycle the settled activated sludge fraction to the anoxic zone. In the A unit an anaerobic zone was interposed between the anoxic and aerobic zones. Unit A with the interposed anaerobic stage, was found to have superior phosphate removal ability.
The methods currently being employed to effect desired nitrification in systems operating in the A/O mode or in the A.sup.2 /O mode, necessitate a change in the operating conditions otherwise advocated for the anaerobic as well as for the aerobic treatment of the mixed liquor. While the attempted modifications in such operating conditions may be found advantageous for enhancing nitrification, they often may be detrimental to maintaining desired sludge properties or to the desired extent of phosphorus removal. Among methods proposed or used for enhancing nitrification in A/O and A.sup.2 /O systems, is that of increasing the solids concentration in the aerobic basin by increasing the amount of sludge recycled. Such method has the drawback of thus reducing the food to biomass ratio (F/M) in the anaerobic zone, with possible subsequent detrimental effects on sludge properties and phosphorus removal.
Another method for inducing an A/O or A.sup.2 /O system to nitrify, is to increase the aerobic influent detention time (IDT). This can be accomplished by providing an increased volume for the aerobic zone. Such volume increase, however, would change the ratio of the IDT in the anaerobic zone to the IDT in the aerobic zone. Decreasing flow of influent to the A/O or A.sup.2 /O system would provide an increase in aerobic IDT, but at the same time would increase the anaerobic IDT, and accordingly would decrease the F/M ratio in the anaerobic zone.
One of the objects of the present invention is to maximize removal of phosphorus in a system operating in the A/O or A.sup.2 /O mode accompanied by desired nitrification, without adversely effecting phosphorus removal or impairing desired sludge properties.